This invention relates to a lamellate polytetrafluoroethylene material that can be formed into an implant where there is an improvement in the surgical handling accompanied with enhanced healing properties due to the novel arrangement of variable porosity regions of polytetrafluoroethylene. This invention relates to materials utilized in the production of devices for in vivo implantation, such as heart valve leaflets, sutures, vascular access devices or any related products, but more particularly relates to vascular grafts, for example, to porous polytetrafluoroethylene prostheses intended for placement or implantation to supplement or replace a segment of a natural, biological blood vessel. It also relates to patches or supports for tissue repair or reinforcement. For simplicity of exposition below, the invention will be discussed solely with relation to an implantable vascular graft, or a liner for a vessel which might, for example, be delivered intraluminally.
Reference is made to commonly-owned U.S. Pat. Nos. 5,433,909 and 5,474,824 which disclose a method and a product made thereby which is an extruded tube of polytetrafluoroethylene having a tailored porosity. The tube is made by extrusion, of a perform having differently-prepared PTFE paste at two or more levels along its radial section, followed by stretching and generally also sintering so as to achieve the desired strength and pore structure in the final product. As described in those patents and elsewhere, porosity may be tailored to achieve certain desirable properties in the structure of nodes and fibrils that affect permeability and various forms of tissue compatibility, such as the promotion or prevention of tissue growth. In particular, the above patents describe a method of fabricating tubes of PTFE material having good mechanical strength, together with a combination of other features including one or more of a large reticulated node structure which enhances tissue growth, a small pore structure which limits weeping of the graft, and different porosities through the thickness portion of the tube wall to achieve desired properties at both surfaces. The aforesaid commonly-owned patents also describe a method of obtaining these properties in a single PTFE tube in which a property such as lubrication level has been consciously made non-uniform.
Other approaches to extruding a porous PTFE tube have involved stacking two preforms of different PTFE materials, or PTFE and a dissimilar material, together and extruding a layered structure.
Still other approaches to incorporating PTFE as the sole or a large portion of a vascular graft have involved numerous constructions. These include constructions wherein an inner tube is surrounded by one or more other layers of tubing, foam or fiber wrapping to enhance its mechanical compliance and, for example, provide direct impermeability, or result in clotting which, after a short time, becomes impermeable. The inner tube is generally formed of PTFE, selected for its highly advantageous biocompatibility properties in the blood path. Various outer layers may consist of fibers either helically wound or electrostatically flocced, films of thin material, tape wrap generally also of thin material, or coatings. Materials used for these layers may also include impermeable polyurethane or other soluble polymer coatings, emulsions and also PTFE films.
These composite structures are in some ways similar to the earlier generation of fabric grafts made of woven or knitted Dacron or the like, and each represents an attempt to address or optimize some of the various constraints encountered in trying to replace a vessel with material which is strong, capable of long term patency and has some degree of tissue compatibility.
In general, however, conventional vascular grafts manufactured from porous polytetrafluoroethylene have limitations in surgical handling and healing.
Presently, many vascular grafts exhibit some degree of weeping or blood loss during implantation. A variety of factors effect this surgical complication, one being prewetting of vascular grafts with heparinized saline or antibiotics to render the surface thrombus and infection resistant. Prewetting of the graft results in a reduction of the hydrophobic properties with an effective increase in permeability. Cohesion of platelets and adhesion of fibrin in the graft wall can initiate the coagulation cascade resulting in thrombus formation. The thrombi are responsible for the formation of emboli in tubular prosthesis with small diameters.
Native arteries and veins have a common pattern of organization made up of three layers: an internal intima, surrounded by a media, and then an external adventitia. Each of these layers has a predominant structure and cell-type. The walls of arteries are built of elastin, collagen, a non-fibrous glucosaminoglycan-rich matrix and smooth muscle cells. The microscopic structure of the artery wall correlates with the function of the various wall-layers and components.
Several studies support the belief that there is a net transport of macromolecules across the arterial wall. The transport process is controlled by diffusion, convention, and forces. Convection is associated with the hydraulic flux resulting from pressure or osmotic differences across the arterial wall. Diffusion occurs in response to a concentration driving force.
While a number of vascular grafts, or processes for preparing the same, provide for a stronger graft, such grafts do not generally possess a differential permeability effective to achieve enhanced healing and tissue ingrowth, and at the same time offer improved surgical handling.
There is a need for an in vivo implantable material device, and in particular vascular grafts which are formed as a lamellate structure that mimics the natural artery with differential cross-section permeability composed of collagen and elastin and is acceptable to the surrounding tissue.
It remains desirable to provide prostheses or material having enhanced tissue compatibility or long term patency or growth compatibility characteristics.